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Monday, October 26, 2015

A reader pointed out that I forgot to provide links to the European Space Agency studies of possible contributions to NASA's Europa mission. You can find a press article on NASA's original offer to ESA here and a link to the studies here.I've also put a poll in the upper right corner of the blog page where you can show your preference for which of NASA's Discovery missions you would like to see selected.

Saturday, October 24, 2015

Last year, NASA’s managers invited the European Space Agency (ESA) to
propose a small spacecraft to explore the Jovian system.The small craft would be carried to Jupiter
by NASA’s own, large Europa multi-flyby spacecraft.This daughter mission could add to the
exploration of Europa or study another target within the Jovian system.

ESA has recently posted the results of studies for two possible spacecraft
that might be carried by NASA’s Europa spacecraft to the Jupiter system. One would land on Europa and the other would
fly by the volcanic moon Io.While these
were concept studies and not an actual proposal from ESA to NASA, they give an
idea of the possible capabilities and limitations on an ESA contribution.

In the coming year, Europe’s scientists can make actual proposals for
spacecraft to be added to NASA’s mission.They can do so through ESA’s competition to select it’s fifth medium sized
(~550M Euros or somewhat more in dollars at the current exchange rate) science
mission.Proposals for the Europa
mission will be pitted against other planetary and astrophysics missions.However, because NASA would cover the costs
of launch and delivery to the Jupiter system, proposals for the Europa mission
could have a leg up in the competition.

A small ESA spacecraft would need to find a scientific angle not
already taken by its larger cousins. Next year, NASA’s Juno spacecraft will study the Jupiter itself from
just a few thousand kilometers above the top of the cloud deck.ESA’s JUICE spacecraft will arrive in the
late 2020s to study Jupiter from afar, flyby Europa and Callisto multiple
times, and then orbit Ganymede.NASA
Europa Mission (apparently no longer called the Europa Clipper) will flyby
Europa 45 times as well as flyby Ganymede and Callisto.These missions will carry suites of extensive
and highly capable instruments.

Under NASA’s proposal, the American space agency is reserving space and
250 kg of mass to host the European spacecraft.(NASA is also separately reserving mass for the equipment to connect the
two spacecraft.)

So if you had a ride to the Jupiter system for 250 kg, what could you do with it?

Two obvious possibilities were mentioned at the time that the offer was
announced: build a small lander for Europa or a small spacecraft that would fly
through any plumes erupting from Europa’s surface.The ESA team looked at both.

For a lander, the European study group considered a type of hard lander
known as a penetrator.Shaped roughly
like a cannon shell, penetrators smack into the surface and then travel a few
meters into it before coming to a stop.(Think of a bullet shot into the ground.The friction between the bullet’s surface with the soil slows and
eventually stops the bullet.)

Concept illustration of a Europa
penetrator (blue) embedded in the icy surface.An after body (green) remains on the surface with the antenna to
communicate with NASA’s Europa mission spacecraft.A cable would connect the two sections.Credit: ESA

Penetrators have been used on the Earth to deploy sensors from planes into
remote locations such as ice shelves.Penetrators
have the advantage of not requiring expensive landing systems – the penetration
into the surface supplies the braking.A
unique advantage on Europa is that once buried, the surrounding ice protects
the probe from the radiation fields around Europa.

The concept studied would use the penetrator to deliver two sets of
instruments in to the ice.The first set
would study the chemistry of the ice and materials within it.A habitability package would use chemical
reactions to look for conditions such as pH consistent with possible life, a
mass spectrometer would analysis the composition, and a microscope would image
the sample.A seismometer would record
Europa-quakes to study the level of activity within the icy crust and to gather
clues about its structure.

The penetrator with its deployment
stage.Credit: ESA

The penetrator would be delivered by a deployment stage that would
essentially be a small spacecraft with a substantial retrorocket.The main NASA spacecraft would target the
daughter craft to pass just 35 km above the landing zone.Just before that distance, the European spacecraft
would fire its rocket, reducing its speed to zero relative to the Europan
surface below.The delivery spacecraft
with the attached penetrator then begin their free fall to the surface.The delivery craft would have 231 seconds to
reorient itself so that the penetrator points straight down and then release
it.NASA’s Europa spacecraft passing overhead
would have a short few minutes to listen for a radio confirmation that the
landing succeeded.

The penetrator deployment.SRM burn is the rocket burn that removes all
velocity between the penetrator and the surface of Europa.The diagram uses the older name ‘Clipper’ for
the NASA spacecraft.Credit: ESA

Approximately ten days later, the NASA spacecraft would return to
Europa and receive the data collected by the penetrator’s instruments.

While the penetrator concept is exciting, the devils are in the
details.Planetary penetrator missions
have been studied for many potential missions.Russia launched two on a mission intended for Mars but which never left
Earth orbit.NASA delivered two tiny
penetrators to Mars, but they were never heard from after they were released by
their carrier spacecraft.Japan spent
years developing penetrators for the moon but eventually cancelled the project
because of development problems.

A key problem with penetrators is that they need relatively flat landing
sites for successful landings.Europa’s
surface is covered in slopes and rough terrain.Also, Penetrators are built to tolerate high vertical velocities, but
any lateral velocity can destroy the payload inside.(Put another way, engineers can design for
high G’s in one direction, but it’s hard to design for all directions.)This means that the retro rocket must
successfully kill all but the smallest lateral movement so the penetrator moves
only vertically during its descent.

Another problem with penetrators is that the space inside is small and
any instruments must be built to withstand high impact forces.As a result, there’s usually significant instrument
development required.The penetrator
concept report states that the instruments to study the chemistry of the ice
are at a low state of technical readiness for use in a penetrator.

Issues such as these have kept penetrators as a great idea that has
never been matured enough to become a reliable tool for planetary exploration.

My take on the report descripting the penetrator concept is that
delivering a penetrator for Europa appears to be a high risk possibility both
for completing the development in time for a launch and for actual
delivery.Another significant problem is
that the concept craft would have a mass greater than 300 kg, well above the
250 kg NASA is offering.

The other concept studied by ESA’s engineers would be a daughter
spacecraft that would be a straightforward use of existing technologies.The original idea was for a small spacecraft
that could fly through plumes erupting from Europa.This idea seems to have lost its appeal.First, diligent searches have failed to
confirm the original observation of a possible plume (which was made at the
limits of detectability).Second, NASA’s
Europa spacecraft is highly capable with cutting edge instruments, and it could
fly through any plumes itself.

Between the Juno, JUICE, and Europa missions, almost all of the Jupiter
system is already targeted for detailed study.An exception, though, is the extremely volcanic moon Io that sits deep
within Jupiter’s radiation field.That
moon became the target for the second study.

In this concept, NASA’s craft would release the European orbiter
shortly after the two jointly enter Jovian orbit.The ESA craft then would fire its own engine
to lower the perijove of its orbit to encounter Io at least twice.

The Io flyby spacecraft would be so small
that this top view engineering diagram uses millimeters for its length
units.The triangular main spacecraft
body would be below the center main antenna dish.Credit ESA.

The Io flyby spacecraft would carry four instruments.A multi-color camera would image the surface
at resolutions ranging from 2.2 km to 18 m per pixel, a thermal mapper would
identify and measure the temperature of hotspots at resolutions ranging from 30
km to 50 m per pixel, a mass spectrometer would measure the composition of ions
and particles ejected from Io, and a magnetometer would study the magnetic
field around Io.

The trajectory for each of the two Io
flybys (second flyby shown here) would be chosen so that the ground tracks pass
over several areas of known volcanic activity so that these areas could be
studied in high resolution.The first
flyby would pass 500 km above the surface at closest approach, the second flyby
100 km.Credit ESA

Jupiter’s radiation is strongest in the plane of the equator where its
major moons, including Io, orbit.Since
the ESA craft would be released from the NASA craft in an equatorial orbit, the
ESA mission would receive the full radiation baking.ESA’s spacecraft would have to traverse this
radiation on both the inbound and outbound legs of its passes.(A larger, fully dedicated Io mission such as
the proposed Io Volcano Observer proposed for NASA’s Discovery program, would
use a polar Jovian orbit instead, limiting its radiation exposure.)The tiny ESA spacecraft potentially could
perform more than two flybys if the harsh radiation close to Jupiter degrades
the craft’s electronics more slowly than expected.

The Io spacecraft study report does suggest that if the idea of an Io
spacecraft is pursued, that the option of releasing it before the Jupiter orbit
insertion burn is done should be considered.That way, the Io spacecraft could do its own insertion burn and enter a
Jovian polar orbit to reduce radiation exposure.

The Io flyby concept studies would come in on the heavy side by missing
the 250 kg target mass by 17 kg.To get
that close, the concept design had accept a “more risky operational scheme”,
that is, reduce backup systems and capabilities to minimize weight.

Either of these missions would be an exciting addition to the already
planned JUICE and Europa missions.The
Io flyby craft seems to be less risky both to design and to fly.

When NASA first announced that it would offer room and mass for an ESA
probe, ESA’s managers said they would let scientific teams propose which
concepts would be considered as part of the next Medium science mission
competition.These two proposals are
proof of concept studies that the proposing teams can use to inform their
proposals.We may be seeing even more
interesting proposals from the science teams.I can think of several alternative probes, and I’m sure the
professionals can think of even more creative ones.

If ESA does contribute a small probe to NASA’s mission, the exploration
of the Jupiter system may be more interesting than it already promises to be.

Saturday, October 3, 2015

Over the
last few months, NASA’s managers have had the tough job of selecting a handful
of proposals for new missions from an outstanding set of 27 proposals. Proposed targets ranged from Venus, our moon,
Mars and its moons, the comets and asteroids, Jupiter’s moon Io, Saturn’s moon
Enceladus, to space telescopes to observe solar system bodies.

In the
end, two Venus and three asteroid missions received the nod to receive $3
million each for further study pending final selections in a year. Launch of the selected mission or missions is
likely for the early 2020s.

The
current competition is to select the 13th and possibly the 14th
missions in NASA’s Discovery mission program.
In the past, missions in this program have orbited Mercury, orbited the
moon, landed on Mars, flown by comets, orbited three asteroids (landing on one),
and searched for exoplanets. This
program lets teams of scientists propose and lead the missions (in partnership
with a NASA or industry partner for engineering expertise). Neat fact about the current finalists: Four
of the five teams are led women.

Costs for
the current competition are capped at $500 million for the spacecraft and
instruments with NASA separately paying for other costs such as the launch. For comparison, this is more or less half the
cost of the New Frontiers Pluto mission, a fifth the cost of the Curiosity Mars
rover, and a quarter the cost of NASA’s planned Europa mission.

In the
early years of the program from approximately the mid-1990s into the early2000s,
NASA regularly selected Discovery missions every two to three years and would
often select two missions at once. Then
budgets became squeezed and the last two mission selections were stretched out
to every five years with a single selection each. (The GRAIL orbiters selected in 2007 studied
the moon for a year beginning in late 2011, and the Mars InSight geophysical
mission selected in 2012 will launch next year).

In this
current competition, NASA believes it may be able to again
select two missions, which would be staggered in their launch and
development costs. NASA’s managers
haven’t stated why they now hope to select two missions instead of the
originally planned one. Their budget
forecasts may look rosier than previously expected. It may be because the cost of the Discovery
competitions to NASA and the proposal teams is high enough that the agency’s
managers want to limit their frequency by delaying the next one and selecting
two at once. Or the estimated cost of
some of the missions selected as finalists could be implemented under the cost
cap.

NASA
evaluates the proposals
based on two sets of criteria, and the competition is tough. Teams of scientists rank each of the
proposals based on its scientific potential to help us understand the solar
system. Separately, teams of engineers
and budget analysts scrutinize the implementation details to determine whether
each proposal likely could be implemented within the budget and schedule. The finalists are selected from among the
proposals that rank highest on both sets of criteria.

The primary goal for the DAVINCI mission would be to make high priority measurements of the composition of Venus’ atmosphere.Credit: NASA

The DAVINCI (Deep Atmosphere Venus
Investigation of Noble gases, Chemistry, and Imaging) proposal would drop an
instrumented probe into Venus’ atmosphere.
During its descent to the surface, the DAVINCI probe would measure the
composition of the atmosphere’s gases and image the surface from below the
clouds.

The team proposing DAVINCI was one of
the quietest during the competition; while many other teams presented their
proposals in some detail, not even the name of this proposal leaked. A brief
post on the Unmanned Spacelight message board reports that the instruments
would include a mass spectrometer, a tunable laser spectrometer, an atmospheric
structure package, and a visible and near-infrared descent camera.

By looking at papers and conference
proceedings that include the proposal’s Principal Investigator, Lori Glaze with
NASA’s Goddard Spaceflight Center, we can get some ideas about the mission’s scientific
questions.

The composition of a planet’s atmosphere
can reveal much about the planet’s formation, its evolution, and current
geological processes such as surface weathering and volcanic eruptions. A recent conference
abstract that included Dr. Glaze, stated, “A key issue that remains after
more than 50 years of planetary exploration is the formation and evolution of
the atmosphere, particularly in the context of the other terrestrial planets.
Comparing noble gas mixing ratios and isotopes of Venus, Earth, Mars, Jupiter,
and the sun will help determine the timing and extent of atmospheric escape on
Venus, a central process in planetary evolution.” Several research papers that include Dr.
Glaze also discuss how volcanoes on Venus would release gases such as sulfur
dioxide into the atmosphere that would indicate whether or not Venus has
currently active volcanism.

According to a blog
post on the journal Science’s site, the probe would descend over one of the
planet’s tesserae and would image the terrain below as it fell. These crumpled highlands may be remnants of
ancient crust on Venus. Images as the
probe falls below Venus’ clouds could provide clues about the origins of these
mysterious regions and the evolution of the planet’s surface.

Both the DAVINCI and the VERITAS missions would search for current volcanic activity on Venus.Credit: ESA - AOES Medialab

DAVINCI is an example of a mission in
which a few key measurements focus on selected critical science questions.The entire descent would likely take less
than an hour.The data from the
atmospheric composition measurements might be just a few megabytes of
data.(A study of an atmospheric Saturn
probe to study composition listed the total data as less than 2M bytes, less
than the size of a high resolution image from my personal camera.)The images collected by the probe’s camera
might be a few megabytes to gigabytes.By comparison, orbiter missions at planets can return terabytes of data.

However, detailed composition
measurements of planetary atmospheres is a high priority for planetary research
because they can reveal details about the formation of each planet and its
subsequent evolution. The Pioneer Venus
probe from the 1970s lacked the resolution for key measurements. We have high resolution measurements of Mars’
atmosphere from various landers and from Jupiter from the Galileo atmospheric
probe. Obtaining high resolution
composition measurements from Venus (as well as Saturn, Uranus, and Neptune)
has been a high priority for planetary scientists for decades. Each high resolution set of measurements for
a new world provides a new piece of the puzzle to help us understand how the
solar system formed.

Where
the DAVINCI mission would focus on specific scientific questions and gather a
small amount of critical data, the other finalist Venus mission takes the
opposite approach. The VERITAS
(Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) mission
would remap Venus’s surface with radar and conduct the first global mapping of
its surface composition. Venus’s surface
was previously mapped in low to moderate resolution by NASA’s Magellan mission
in the early 1990s.

The VERITAS mission would map with radar and infrared spectroscopy.Credit: NASA/JPL-CALTECH

The mission’s VISAR (Venus
Interferometric Synthetic Aperture Radar) would map the surface in three
ways.First, it would create images of
the surface at 30 m resolution globally and 15 m in selected regions compared
to Magellan’s 280 m to 120 m resolution.Second, it would measure elevations to create a topographic map at 250 m
resolution compared to Magellan’s 15 to 27 km resolution.And third, it would make repeat measurements
in what’s known as an interferometric mode to spot tiny changes in relative elevations
that could indicate surface movement from a seismic event or the swelling of a volcano.

VERTITAS could greatly improve the resolution of images of Venus surface
(top) and topography (bottom) as shown by these simulations based on
terrestrial data (Hawaii and Iceland; the topographic image is from a paper
discussing general improvements possible by a new mission and VERITAS’s actual
resolution may be different). Credit: NASA/JPL-CALTECH;
Decadal Survey White Paper, NASA

A second
instrument, the German Venus Emissivity
Mapper (VEM), would study the planet’s thermal emissions for composition
studies. The Galileo and Venus Express missions’ instruments discovered narrow
spectral windows where thermal emissions can be transmitted through the
otherwise opaque global cloud cover. These few windows would give the VEM
instrument the ability to map the surface in six spectral bands to identify thermal
hotspots that could indicate areas of current volcanic activity, map differences
in the surface composition, and detect changes in key atmospheric gases that could
indicate the eruption of a volcano. Because Venus’ thick atmosphere would scatter
the light, the surface resolution of VEM would be low, perhaps around 50 km. The
recently completed Venus Express mission carried out some measurements using
this technique, but its instrument wasn’t optimized for measurements using
these spectral bands and covered only the southern hemisphere.

While the
DIVINCI mission would focus on a few critical measurements and would produce a
relatively small data volume, the VERITAS mission would make multiple and
repeated measurements over the surface of a large world. A proposal for a similar European mission
said that it would return hundreds of terabytes of data; VERITAS likely would
do the same. Researchers could use this
database to enable hundreds of studies.
A poster on the VERITAS mission (unfortunately no longer available on
the web) listed a few:

Origin and Evolution: How did Venus
diverge from Earth?

• Determine if tesserae are remnants of
an earlier wetter past

• Search for past tectonic or cratered
surface beneath the plains

Venus as a Terrestrial Planet: What
processes shape the planet?

• Determine how and when Venus was
resurfaced

• Estimate lithospheric thickness
variations with time

•
Identify sources and rates of recent and active volcanism

NASA also
has the option for a technology demonstration for the VERITAS mission that
would partially address the DAVINCI composition measurement goals. If funded, the tiny Cupid’s
Arrow cubesat would be released by the main spacecraft and would skim the
edges of the outer atmosphere to reach below the homopause where the
atmospheric gases are well mixed. A miniaturized
mass spectrometer would measure ratios of key noble gases that provide
clues to the formation and evolution of Venus.

These two
Venus missions illustrate the different types of missions needed to explore the
solar system. The study of Venus
requires both and eventually both will fly.

The three
finalist proposals to study asteroids provide another example of the
complementary types of missions needed study the solar system. Asteroids are remnants of small proto-worlds from
the early formation of the solar system and differ in location and
composition. Our spacecraft will never
visit more than a few of the millions of these bodies believed to orbit the sun. Scientists instead use telescopes to gather a
few facts on many bodies to enable statistical studies, make brief flybys of a
small number to flesh out the statistics, and make prolonged visits at a very
few for in-depth studies (and to return samples from a few).

The NEOCam space telescope. Credit: NASA/JPL-CALTECH

The Near-Earth Object Camera (NEOCam) mission
would launch the first space telescope dedicated to observing asteroids. Its focus would be on the population of
asteroids that, as its name states, approach near to our own world. By making measurements in two infrared
channels for each of the tens of thousands of near-Earth asteroids, the science
team will be able estimate sizes, shapes, composition, orbit about the sun, and
rotation for each body. While the
information on any one body will be limited, the statistical analysis made
possible on a data set of tens of thousands of bodies would enable scientists
to explore the dynamics, origins, and fate of these populations. (Past or future observations of many of the
same bodies in other wavelengths of light, particularly the visible, will add
valuable complimentary information.) During
its survey, NEOCam also would observe approximately a million main belt
asteroids and discover perhaps a thousand new comets, extending the usefulness
of the statistics derived from its data.

However, the
scientific study of these asteroids are only a part of the mission’s
justification. Some proportion of
near-Earth asteroids will eventually strike our world. Finding even one that threatens the Earth in
the next few decades would justify the mission by itself. Some of the objects discovered also could
become targets of future robotic or human missions.

Summary of the Lucy mission from the proposal’s factsheet. Credit: SwRI

The Lucy
mission would follow the second strategy for asteroid exploration, brief flybys
of a number of asteroids. The mission’s
proposers have reused the name of one of the most famous fossils from human
paleontology to emphasize that the spacecraft would focus on a fossil
population of asteroids that may hold the potential to illuminate the ancient
history of the solar system. It would
study the Trojan asteroids that share Jupiter’s orbit, either preceding (the “Greek”
camp in L4 Lagrangian orbits) or trailing
(the “Trojan” camp in L5 Lagrangian orbits) the giant planet. Telescope observations suggest these bodies
have primitive compositions, several of which don’t appear to be represented in
our meteorite collections and that haven’t yet been visited by spacecraft.

The
origin of this asteroid population is a mystery, and its solution would tell
scientists much about the dynamics of the young solar system. We now believe that the orbits of the giant
planets migrated in toward the sun and then out again soon after their
formation. In the process, they
scattered the tiny asteroids and comets hither and thither. One set of theories holds that the migration
brought in groups of asteroids from throughout the outer solar system into
Trojan orbits with Jupiter. Another theory
suggests that the Trojans originated in the same region as Jupiter and followed
it in its movements and are therefore samples of conditions where Jupiter formed. Either way – and it’s possible that the
present population represents a mixture of sources – these bodies hold clues to
conditions and processes from the infancy of our solar system.

The
creativity of the Lucy mission is that its proposers found a set of trajectories
that over 11 years allow flybys of two Trojan asteroids in the L4 swarm and a
binary Trojan system in the L5 swarm with a bonus flyby of a main belt asteroid. The three Trojan encounters would sample a
diversity of compositions, the C-, P-, and D-types.

This
mission looks to the New Horizon Pluto mission for two of its instruments with
copies of that mission’s LORRI high resolution camera and the RALPH color
camera and imaging spectrometer. Another infrared spectrometer would draw
on instrument heritage from Mars orbiters and the upcoming OSIRIX-REx asteroid
sample return. Tracking of the spacecraft’s radio signal would provide
information on each asteroids mass and therefore density which provides clues
to their composition and to whether
they are solid objects or rubble piles.

The Psyche spacecraft above an artist’s concept of what the surface of a metallic asteroid might look like.

The third asteroid mission would make an extended study of a
single asteroid. The asteroid16-Psyche
is unique among the larger asteroids in having a composition that appears to be
largely metallic. Understanding how this
world came to be is one of the goals for this mission. Psyche could be an
asteroid in which repeated collisions chipped off the crust and mantle, leaving
the core a naked body. It could
be the remnant of the collision of two protoplanets that shattered and expelled
the core of the smaller body to become Psyche. Or Psyche could have formed so close to the
early sun that only metals (and some silicates) could have condensed from the
nebula; the later migration of the giant planets could have moved it to its
present location in the asteroid belt.
In either of the first two cases, we’d get our first look at material
from one of the most inaccessible locations in the solar system – the deep core
of a rocky world. In the third case, we’d
see the result of a new class of worlds that formed very close to the sun.

In its implementation, the Psyche mission
would be much like the current Dawn mission to the larger asteroids Vesta and
Ceres. Solar electric ion engines would
slowly propel it to its destination where the spacecraft would orbit the asteroid
for long term studies. A combination of
cameras and spectrometers would image the surface and map its composition while
radio tracking would reveal its interior structure.

From the original 27 proposals, these five are the ones that NASA’s
managers determined have the best combination of scientific appeal and low
implementation risk. For the next
several months, the proposal teams will be consumed with fleshing out the design
of their missions. Then the space agency
scrutinize the enhanced proposals to select one or two to fly.

If either DAVINCI or Lucy isn’t selected, scientists interested in
their studies will get a quick chance to try again. NASA has another program for scientist-led
missions, the New Frontiers program, which flies missions costing approximately
twice what Discovery missions cost. For
this program, NASA accepts proposals from a list of pre-selected, high priority
concepts. One of those concepts is for a
Venus atmospheric probe and lander that would replicate the DAVINCI atmospheric
studies and also provide measurements studies from the surface. A second concept would be for a mission that
would orbit a Trojan asteroid and possibly fly by one or two others. (The other candidate concepts are for a lunar
sample return, a comet sample return, and a Saturn atmospheric probe.) The competition to select the next New
Frontiers mission is scheduled to begin immediately after the selection of the
next Discovery mission(s) next September.

I’m personally hoping that the agency selects both a Venus and an
asteroid mission from the current Discovery competition. The greatest strength of the Discovery
program has been missions to a diversity of worlds.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.